Method and apparatus for plasma treatment of substrates

ABSTRACT

A method and apparatus for efficiently generating a gaseous plasma particularly for the treatment of substrates. A radio frequency electrical signal is applied to two electrodes disposed exteriorly of an electrically insulative, gas impervious envelope. A central passage extends into the envelope and one electrode is disposed in the central passage. The electrodes are separated at least in part by the envelope and the radio frequency signal applied to the electrodes excites the gas within the envelope to thereby generate a gaseous plasma therein. The gas conditions within the envelope differ from the gas conditions exteriorly thereof and the amplitude of the radio frequency signal is insufficient to generate a plasma outside the chamber defined by the envelope. Since the plasma does not contact the electrodes, efficiency is maximized and the plasma is not contaminated by the electrodes. In addition, the surface areas of the electrodes differ substantially thereby creating a plasma within the envelope which varies in concentration in a predetermined manner, with the concentration being greatest near the center of the envelope. A substrate may therefore be contacted by varying plasma concentration as it passes through the envelope and the outer wall of the envelope is not contaminated by the plasma. A vacuum lock for preventing gas leakage into the envelope is also disclosed.

United States Patent 1191 Boom [ 1 Mar. 27, 11973 METHOD AND APPARATUS FOR PLASMA TREATMENT OF SUBSTRATES [75] Inventor: Abraham A. Boom, Martinsville, NJ.

[73] .Assignee: Celanese Corporation, New York, N.Y.

[22] Filed: Aug. 12, 1971 [21] Appl. No.: 171,282

Primary ExaminerF. C. Edmundson Att0rney-Thomas J. Morgan et al.

[57] ABSTRACT A method and apparatus for efficiently generating a gaseous plasma particularly for the treatment of substrates. A radio frequency electrical signal is applied to two electrodes disposed exteriorly of an electrically insulative, gas impervious envelope. A central passage extends into the envelope and oneelectrode is disposed in the central passage. The electrodes are separated at least in part by the envelope and the radio frequency signal applied to the electrodes excites the gas within the envelope to thereby generate a gaseous plasma therein. The gas conditions within the envelope differ from the gas conditions exteriorly thereof and the amplitude of the radio frequency signal is insufficient to generate a plasma outside the chamber defined by the envelope. Since the plasma does not contact the electrodes, efficiency is maximized and the plasma is not contaminated by the electrodes. In addition, the surface areas of the electrodes differ substantially thereby creating a plasma within the envelope which varies in concentration in a predetermined manner, with the concentration being greatest near the center of the envelope. A substrate may therefore be contacted by varying plasma concentration as it passes through the envelope and the outer wall of the envelope is not contaminated by the plasma. A vacuum lock for preventing gas leakage into the envelope is also disclosed.

18 Claims, 8 Drawing Figures GENERATOR PATENIEDmzmn 3,723,289

SHEET 10F 2 GENERATOR INVENTOR A. A. BOOM PATEHTEDMARZYISYS 3 723,2 9

SHEET 2 BF 2 GENERATOR FIG. 5

METHOD AND APPARATUS FOR PLASMA TREATMENT OF SUBSTRATES BACKGROUND OF THE INVENTION The present invention relates to a method and apparatus for treating substrates and specifically to a method and apparatus for more efficiently generating a plasma for the treatment of substrates and for subjecting a substrate to varying plasma concentrations during the treatment thereof.

Various substrates have been treated in gaseous plasmas to obtain desired substrate characteristics. An example of one such process is disclosed and claimed in US. Pat. application Ser. No. 93,350 filed Nov. 27, 1970 by Forschirm et al. for Surface Modification of Organic Polymeric Materials and assigned to the assignee of the present invention. In the Forschirm et al. application, an organic polymeric fiber is introduced into a gaseous plasma for modification of the surface thereof. For example, a polymeric continuous filament yarn may be exposed to a gaseous plasma formed by exciting argon or other suitable gases at a pressure of about 2 mm Hg. with a 4 megahertz, 1000 watt radio frequency signal, to modify the yarn to obtain desirable surface characteristics.

Another process for treating fibers in a gaseous plasma is disclosed and claimed in US. Pat. application Ser. No. 88,358 filed Nov. 10, 1970, for Vapor Phase Boron Deposition by Pulse Discharge" by Kenneth C. Hou and assigned to the assignee of the present inven tion. In the Hon process, a boron coating is deposited on a suitable substrate by generating a boron-hydrogen excited gas species or plasma and contacting the substrate with the plasma. The plasma is generated by applying pulsed high frequency electrical power to a gaseous mixture of boron and hydrogen at a pressure of about 1 to 3 atmospheres. For example, a 3000 volt peak-to-peak a.c. signal having a frequency of 13.6 megahertz in pulses of 100 microseconds duration at a pulse repetition rate of l kilohertz may be utilized to excite the gaseous mixture within a coating zone into which the substrate is introduced to provide a smooth, firmly adhering layer of boron l to 2 mils in thickness.

Carbonaceous fibrous materials have been treated in plasmas as is described in United States patent application Ser.'No. 99,169 filed Dec. 17, 1970, for Surface Modification of Carbon Fibers, by KennethC. Hou and assigned to the assignee of the present invention. In this Hou process, a carbonaceous fibrous material is contacted for a brief time with an excited gas species generated by applying high frequency electrical energy in pulsed form to a gaseous mixture of a monotonic inert gas and a surface modification gas. For example, a carbonaceous yarn may be passed through a gaseous mixture of helium and oxygen wherein the oxygen is present in the mixture in a concentration of about 0.5 percent by weight. A 3 kilovolt peak-to-peak a.c. signal having a frequency of 13.56 megahertz may be utilized to excite the gaseous mixture thereby contacting the yarn with the excited gas species to modify the surface thereof.

In the above-described processes, the excited gas species or plasma is generated by electrically exciting the gas or gaseous mixture. For example, energy may be imparted to gas capacitively and a plasma thereby generated. The plasma is highly electrically conductive and a high conduction current flows between the capacitor plates or electrodes because of the resultant decrease in the electrical resistance of the gas between the electrodes. In the treatment of fibers for commercial uses, the cost of the power required to generate the plasma becomes an important factor.

It is desirable to keep current flow to a minimum since the efficiency of the processes decreases and the cost of treating fibers increases with an increase in current. in addition, the amount of wasted power in transmission lines will be reduced as current requirements decrease. Moreover, by providing control of the location of the major concentration of the generated plasma, the plasma may be utilized in a more efficient manner.

The cost of treating substrates may also be dependent upon the length of time during which a reaction chamber may be operated without shutdown for maintenance. It may be necessary to frequently change the gas within the chamber if the gas is contaminated by the electrodes. Also, the useful life of the reaction chamber may be adversely affected by material buildup on the walls thereof during the treatment operation.

It is therefore an object of the present invention to provide a novel method and apparatus for more efficiently generating a plasma.

It is a further object of the present invention to provide a novel method and apparatus for generating a plasma within a reaction chamber for the treatment of fibers.

It is another object of the present invention to provide a novel method and apparatus for reducing the current flow between electrodes in a capacitive plasma generator, and particularly in chambers adapted for the treatment of fibers.

It is still another object of the present invention to provide a novel method and electrical plasma reaction chamber wherein contamination of the plasma by the electrodes is prevented.

It is yet a further object of the present invention to provide a novel method and electrical plasma reaction chamber wherein contamination of the interior walls defining the chamber is minimized.

In some applications utilizing the present invention, itis desirable to selectively expose a substrate to a plasma to achieve selectable surface modification or coating of the substrate. For example, the amount of time during which the substrate is exposed to the plasma may be selectively varied to provide the desired end product. This may be accomplished through control of the speed at which the substrate passes through the plasma, assuming that other conditions remain constant.

Moreover, other conditions to which the substrate is subjected within a plasma reaction chamber may have an effect on the resultant treated substrate. The substrate may, for example, be adversely affected by excess heat or the sudden exposure to high temperatures. It may therefore be desirable to expose the substrate to the plasma in a controllable manner.

It is therefore yet another object of the present invention to provide a novel method and apparatus for selectively exposing a substrate to a plasma.

It is still a further object of the present invention to provide a novel method and apparatus for selectively exposing the substrate to varying concentrations of a plasma within a reaction chamber.

In addition to the above, it is often desirable to maintain the pressure of the gas within a reaction chamber at a pressure lower than that of the gas outside the chamber. The pressure differential between the interior and exterior of the chamber may create problems in introducing the substrate into the chamber. This may be a particular problem where expensive and/or harmful gases are utilized for treatment of the substrates since the escape of these gases or the leakage of undesired gases into the chamber may increase the cost of the process, result in hazardous working conditions, and/or adversely affect the treatment of the substrate,

It is therefore an object of the present invention to provide a novel method and apparatus for passing a substrate from a zone at one pressure into a zone at another pressure without substantial leakage therebetween.

These and other objects of the present invention will become apparent to one skilled in the art to which the invention pertains from a perusal of the following description when read in conjunction with the appended drawings.

THE DRAWINGS FIG. 1 is a schematic representation of a reaction chamber embodying the present invention;

FIG. 2 is a view in cross section of the reaction chamber of FIG. 1, taken along the line 2-2;

FIG. 3 is a schematic representation of the reaction chamber of FIG. 1 with a substrate being treated therein;

FIG. 4 is a view in cross section of the reaction chamber of FIG. 3, taken along the line 44;

FIG. 5 is a schematic representation of a reaction chamber similar to the chamber shown in FIG. 3 with a plurality of substrates being treated therein;

FIG. 6 is a view in partial cross section of the reaction chamber of FIG. 5 illustrating the vacuum lock of the present invention; and,

FIGS. 7A and 7B are end views of the vacuum lock of FIG. 6, taken along the line 77 thereof, and illustrate two of the alternative shapes which the vacuum lock may have.

DETAILED DESCRIPTION Referring to FIGS. 1 and 2 wherein a preferred embodiment of a reaction chamber constructed in accordance with the present invention is illustrated, a reaction chamber 10 is formed by a substantially gas impervious, generally electrically non-conductive or insulative envelope 12 into which a central passage 14 extends. An electrode 16 extends into the central passage 14 and is isolated from the chamber 10 by the radially inward wall of the envelope 12. An electrode 18 is disposed radially outward of the envelope 12, and is separated at least in part from the centrally disposed electrode by at least a portion of the envelope 12, thereby defining an area within the envelope 12, i.e., at least a portion of the chamber 10, which is disposed between the electrodes 16 and 18.

A high frequency electrical potential is applied between the electrodes 16 and 18 from a suitable source such as a variable frequency and amplitude,

radio frequency (RF) generator 20 to thereby subject the chamber as defined by the envelope 12 between the electrodes 16 and 18 to a selectable time varying electrical field. A suitable fill tube 22 may be provided communicating with the chamber 10 through the envelope l2 and having a valve or other suitable closure means 24 therein to selectively control the nature and pressure of the gas within the envelope 12.

With continued reference to FIGS. 1 and 2, the envelope 12 defining the chamber 10 preferably comprises an outer elongated hollow glass cylindrical member 26, an inner elongated hollow glass cylindrical member 28, and apertured end plates 30 and 32 sealed therebetween in a suitable conventional manner. The cylindrical member 28 illustrated is substantially coextensive with the member 26 and is disposed in telescoping relationship thereto coaxially within the member 26 to define a chamber annular in cross section as is shown in FIG. 2.

The central electrode 16 is preferably an elongated metallic cylindrical member, e.g., a wire, telescoped within the central passage 14, but may be hollow. The outer electrode 18 is preferably a hollow cylindrical electrically conductive member circumferentially disposed round at least a portion of the insulative member 26 and may, for example, be a metallic foil conformed to the radially outer surface of the envelope. The central electrode 16 preferably extends axially into the central passage 14 sufficiently so that an elongated annular portion of the chamber 10 is located between the electrodes 16 and 18.

The application ofa potential between the electrodes 16 and 18 creates an electric field between these electrodes, as is indicated by the lines 34 in FIG. 2. The electrode configuration, i.e., the relative positions of the electrodes and the relative dimensions thereof, cause the electric field to be more concentrated or dense in the vicinity of the central electrode 16 near the axis of the annular chamber 10.

If the intensity of the electric field is sufficient, the gas in the chamber 10 will be excited sufficiently to create a gaseous plasma in the chamber. The plasma generally comprises highly reactive species such as ions, electrons and neutral fragmented particles in highly excited states. Since the exciting of the gas by the electric field creates the plasma, the plasma concentration or density generally conforms to the electric field concentration or density. Thus, the concentration or density of the plasma generated within the gas impervious envelope 12 varies between the outer cylindrical member 26 and the inner cylindrical member 28 in a manner related to the electric field concentration of density.

The plasma is thereby concentrated around the inner cylindrical member 28 rather than being dispersed evenly throughout the chamber 10. This central concentration permits more efficient utilization of the plasma for treating substrates and permits selective exposure of the substrate to the plasma as will hereinafter be described. In addition, this central concentration of the plasma prevents excessive buildup of material on the inner wall of the outer cylindrical member 26.

The relationship between the gas conditions within the envelope l2 and the gas conditions exteriorly thereof is desirably such that the plasma may be confined to the chamber 10. The electric potential applied to the electrodes 16 and 18 may thus be lower and the current density will be correspondingly less. This desirable relationship may be obtained by utilizing selected gases at predetermined pressures within the chamber 10, while exposing the electrodes outside the envelope 12 to the atmosphere.

By way of example, a monatomic inert gas, such as argon or helium at atmospheric or slightly less than atmospheric pressure may be utilized in the chamber 10.- When the RF signal is applied to the electrodes 16 and 18, a plasma will be more readily generated within the chamber than exteriorly thereof. With the potential of the RF signal applied to the electrodes set at a value corresponding to the potential required to generate a plasma within the chamber 10, but below the potential required to generate a plasma in the vicinity of the electrodes 16 and 118 externally of the chamber 10, the current which flows between the electrodes 16 and 18 will not be appreciably affected by the ion flow within the highly electrically conductive plasma since these electrodes are electrically isolated from the plasma. The plasma within the chamber 10 is not contacted by the electrodes 16 and 18 and therefore not contaminated by the electrodes.

Referring now to FIG. 3, a substrate 36 to be treated within the generated plasma may be introduced into the chamber 10 through a vacuum lock 38 subsequently described in greater detail in connection with FIGS. 6 and 7. The substrate 36 may be passed through the chamber 10 in contact with the plasma therein at a rate determined by the particular treatment process to which the substrate is being subjected. For example, the substrate 36 may be an organic polymeric fiber, such as a thermoplastic or thermosetting polyester, polyamide, cellulosic or polyolefin material, the surface of which is to be treated in the plasma to obtain a particular surface modification as is described in greater detail in the previously discussed US. Pat. application Ser. No. 93,350, by Forschirm et al. The substrate 36 may alternatively be a carbonaceous fibrous material to be treated in the plasma within the chamber 10 as is described in greater detail in the previously discussed US. Pat. application Ser. No. 99,169, by Kenneth C. Hou. In a further application of the present invention to the treatment of substrates, a coating may be deposited on a suitable substrate by generating a suitable gaseous plasma and contacting the substrate with this plasma. A more detailed description of the substrate and gases utilized in one such coating technique may be had by reference to the previously discussed US. Pat. application Ser. No. 88,358, by Kenneth C. I-Iou. The above referenced Forschirm and Hon patent applications are hereby incorporated herein by reference.

The substrate 36 may be introduced into the chamber 10 at an a angle with respect to the central electrode 16 as is illustrated in FIG. 3. The substrate 36 might thereby follow a path generally indicated at 40 which subjects the substrate 36 to varying concentrations of the plasma as it passes through the chamber 10. Alternatively, as is shown in FIG. 5, one or more substrates 36 may be passed through the chamber 10'substantially parallel to the electrodes 16 at a selected radial distance therefrom, thereby permitting the exposure of the substrates 36 to a selected plasma concentration.

Referring now to FIGS. 6 and 7 wherein the vacuum lock 38 of FIGS. 3 and 5 is illustrated in greater detail, a hollow tube 40 sealed to the end plate 30 of the envelope I2 communicates interiorly with the chamber 10 and provides a passage through which the substrate 36 may be introduced into the chamber 10. The substrate 36 may be, for example, a loosely packed fiber bundle through which air leakage ordinarily occurs during the passage thereof between chambers at different pressures.

The tube 40 generally conforms in cross-section to the shape of the substrate, i.e., the bundle of fibers, but is slightly smaller in cross-section than the bundle causing the fibers to be inwardly compressed against each other and against the internal wall of the tube 40. For example, if the fiber bundle is generally circular in cross-section as in FIG. 7(A), the tube 40 may also be circular in cross-section with a slightly smaller diameter than that of the bundle. Likewise, if the bundle is elliptical in cross-section as in FIG. 7(B), the tube 40 preferably conforms to that shape and is scaled down to slightly smaller dimensions.

One end 42 of the tube 40 is flared or funnel-shaped providing a transition zone for gently compressing the fiber bundle without damage thereto. If desired, the tube 40 may also narrow slightly along the length thereofto further compress the fiber bundle during the introduction thereof into the chamber 10. It should be noted that when the substrate is a tightly packed fiber bundle or a single filament substrate, the diameter of the tube 40 may be the same or slightly larger than the substrate to prevent damage thereto.

At least two fluid passages 44 and 46 are spaced along the length of the tube 40 and communicate with the interior thereof. Each of the passages 44 and 46 is connected to associated pressure sources 48 and 50, respectively.

The gas pressure applied to the passage 46 preferably approximates the pressure in the chamber 10, while the pressure applied through the passage 44 is preferably slightly higher than the pressure in the chamber 10, thereby creating a pressure differential along the interior of the tube 40. This pressure differential, together with the mechanical compression of the substrate, prevents gas leakage into the chamber 10 when, for example, the pressure in the chamber 10 is less than the pressure outside the chamber 110.

With the two passages 44 and 46 illustrated in FIG. 6, gas leakage into the chamber 10 is minimized since a very slight pressure differential, e.g., I mm. Hg, can be maintained between the chamber 10 and the passage 46. An even smaller pressure differential between these two points may be obtained by increasing the number of lateral fluid passages 44 and 46, thereby providing even greater gas integrity between the spaces.

SUMMARY OF ADVANTAGES AND SCOPE OF THE INVENTION It is apparent from the description of the invention that numerous advantages result therefrom. For example, the electrodes are isolated from the highly conductive plasma created within the envelope, resulting in greater efficiency as well as greater current control and eliminating contamination of the plasma by the electrodes.

Control of the substrate treatment process is facilitated by the controlled plasma concentration achieved in the present invention. The substrate may be selectively contacted by the proper concentration of plasma by selecting the path which the substrate follows through the generated plasma. Additionally, the plasma is concentrated in one location within the chamber resulting in more efficient substrate treatment and less material buildup on the interior walls of the envelope.

Moreover, continuous substrates may be treated without adverse effects on the conditions within the reaction chamber since isolation is provided between the interior and exterior of the envelope. For example, the substrate may pass from an area at one pressure, into the envelope which may be at another pressure, and then into an area at yet a different pressure without any substantial gas leakage.

The present invention may thus be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore to be considered in all aspects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed is:

1. Apparatus comprising:

electrically insulative means defining a gas impervious envelope having a central passage extending thereinto;

a first electrode disposed exteriorly of said envelope and within said central passage;

a second electrode disposed exteriorly of said envelope and separated at least in part from said first electrode by a portion of said envelope; and,

means for applying a radio frequency electrical signal to said electrodes to thereby generate a plasma within said envelope.

2. The apparatus of claim 1 including means for introducing a substrate into said envelope for exposure of the substrate to the plasma generated therein.

3. The apparatus of claim 2 wherein said substrate comprises a plurality of fibers; and,

wherein said means for introducing a substrate into said envelope includes means defining an elongated passage interiorly communicating at one end with said envelope, said elongated passage being larger in cross-section at the other end thereof than at said one end whereby said plurality of fibers are inwardly compressed against each other and against the internal walls of said elongated passage sufficiently to substantially preserve the gas imperviousness of said envelope during introduction of said substrate into said chamber through said elongated passage.

4. The apparatus of claim 3 wherein said means for introducing a substrate into said envelope further includes a plurality of pressure sources communicating with said elongated passage at spaced points along the length thereof.

5. The apparatus of claim 3 wherein said means for introducing a substrate into said envelope includes a first means for supplying a first pressure and a second means for applying a second pressure, said first and 5 second means communicating with said elongated passage at spaced points along the length thereof.

6. The apparatus of claim 1 wherein the pressure within said envelope is less than the pressure within said central passage; and,

wherein said radio frequency electrical signal has a potential sufficient to generate a plasma between said electrodes within said envelope but insufficient to generate a plasma between said electrodes externally of said envelope.

7. The apparatus of claim 6 wherein said central passage communicates with the atmosphere on opposite sides of said envelope; and,

wherein the pressure within said envelope is less than atmospheric pressure.

8. The apparatus of claim 7 including means for introducing a substrate into said envelope for exposure of said substrate to the plasma generated therein.

9. The apparatus of claim 8 wherein the surface area of one of said electrodes is large relative to the surface area of the other of said electrodes whereby the plasma generated within said envelope is concentrated in the proximity of said other of said electrodes.

10. The apparatus of claim 9 wherein the distance between one of said electrodes and said substrate within said envelope is varied during passage of said substrate through said envelope to expose said substrate to different concentrations of plasma.

11. The apparatus of claim 1 wherein the surface area of one of said electrodes is large relative to the surface area of the other of said electrodes whereby the plasma generated within said envelope is concentrated in the proximity of said other of said electrodes.

12. The apparatus of claim 11 wherein the distance between one of said electrodes and said substrate within said envelope is varied during passage of said substrate through said envelope to expose said substrate to different concentrations of plasma.

13. The apparatus of claim 1 wherein said gas impervious envelope comprises:

a first elongated hollow member;

a second elongated hollow member telescoped within said first member; and,

means for establishing a gas impervious seal between said first and second members at spaced points along the length thereof to define said gas impervious envelope between said members.

14. The apparatus of claim 13 wherein said first 55 member is a hollow cylinder;

wherein said second member is a hollow cylinder substantially coextensive and coaxial with said first member; and,

wherein said second electrode is a hollow cylinder only slightly larger in diameter than said first member.

15. The apparatus of claim 14 wherein said second electrode is a thin layer of metal conformed to the radially outer, external surface of said first member.

16. Apparatus for treating a substrate comprising:

electrically insulative means defining a gastight chamber;

means including first and second electrodes disposed exteriorly of said chamber for generating a high frequency induced plasma in said chamber; and,

means for passing a substrate to be treated through said chamber along a predetermined path, the distance between one of said electrodes and the substrate varying along said path as the substrate passes through said chamber.

17. Apparatus for treating a substrate comprising:

electrically insulative means defining a gastight chamber; I

means including first and second electrodes disposed exteriorly of said chamber for generating a high frequency induced plasma in said chamber, the surface area of one of said electrodes being substantially larger than the surface area of the other of said electrodes to produce a varying concentration of the plasma within said chamber; and, means for passing a substrate to be treated through said chamber. 18. Apparatus for treating a substrate without expo- 5 sure to a high current density comprising:

whereby the current flowing between said electrodes is reduced; and, means for passing a substrate to be treated through said chamber. 

1. Apparatus comprising: electrically insulative means defining a gas impervious envelope having a central passage extending thereinto; a first electrode disposed exteriorly of said envelope and within said central passage; a second electrode disposed exteriorly of said envelope and separated at least in part from said first electrode by a portion of said envelope; and, means for applying a radio frequency electrical signal to said electrodes to thereby generate a plasma within said envelope.
 2. The apparatus of claim 1 including means for introducing a substrate into said envelope for exposure of the substrate to the plasma generated therein.
 3. The apparatus of claim 2 wherein said substrate comprises a plurality of fibers; and, wherein said means for introducing a substrate into said envelope includes means defining an elongated passage interiorly communicating at one end with said envelope, said elongated passage being larger in cross-section at the other end thereof than at said one end whereby said plurality of fibers are inwardly compressed against each other and against the internal walls of said elongated passage sufficiently to substantially preserve the gas imperviousness of said envelope during introduction of said substrate into said chamber through said elongated passage.
 4. The apparatus of claim 3 wherein said means for introducing a substrate into said envelope further includes a plurality of pressure sources communicating with said elongated passage at spaced points along the length thereof.
 5. The apparatus of claim 3 wherein said means for introducing a substrate into said envelope includes a first means for supplying a first pressure and a second means for applying a second pressure, said first and second means communicating with said elongated passage at spaced points along the length thereof.
 6. The apparatus of claim 1 wherein the pressure within said envelope is less than the pressure within said central passage; and, wherein said radio frequency electrical signal has a potential sufficient to generate a plasma between said electrodes within said envelope but insufficient to generate a plasma between said electrodes externally of said envelope.
 7. The apparatus of claim 6 wherein said central passage communicates with the atmosphere on opposite sides of said envelope; and, wherein the pressure within said envelope is less than atmospheric pressure.
 8. The apparatus of claim 7 including means for introducing a substrate into said envelope for exposure of said substrate to the plasma generated therein.
 9. The apparatus of claim 8 wherein the surface area of one of said electrodes is large relative to the surface area of the other of said electrodes whereby the plasma generated within said envelope is concentrated in the proximity of said other of said electrodes.
 10. The apparatus of claim 9 wherein the distance between one of said electrodes and said substrate within said envelope is varied during passage of said substrate through said envelope to expose said substrate to different concentrations of plasma.
 11. The apparatus of claim 1 wherein the surface area of one of said electrodes is large relative to the surface area of the other of said electrodes whereby the plasma generated within said envelope is concentrated in the proximity of said other of said electrodes.
 12. The apparatus of claim 11 wherein the distance between one of said electrodes and said substrate within said envelope is varied during passage of said substrate through said envelope to expose said substrate to different concentrations of plasma.
 13. The apparatus of claim 1 wherein said gas impervious envelope comprises: a first elongated hollow member; a second elongated hollow member telescoped within said first member; and, means for establishing a gas impervious seal between said first and second members at spaced points along the length thereof to define said gas impervious envelope between said members.
 14. The apparatus of claim 13 wherein said first member is a hollow cylinder; wherein said second member is a hollow cylinder substantially coextensive and coaxial with said first member; and, wherein said second electrode is a hollow cylinder only slightly larger in diameter than said first member.
 15. The apparatus of claim 14 wherein said second electrode is a thin layer of metal conformed to the radially outer, external surface of said first member.
 16. Apparatus for treating a substrate comprising: electrically insulative means defining a gastight chamber; means including first and second electrodes disposed exteriorly of said chamber for generatinG a high frequency induced plasma in said chamber; and, means for passing a substrate to be treated through said chamber along a predetermined path, the distance between one of said electrodes and the substrate varying along said path as the substrate passes through said chamber.
 17. Apparatus for treating a substrate comprising: electrically insulative means defining a gastight chamber; means including first and second electrodes disposed exteriorly of said chamber for generating a high frequency induced plasma in said chamber, the surface area of one of said electrodes being substantially larger than the surface area of the other of said electrodes to produce a varying concentration of the plasma within said chamber; and, means for passing a substrate to be treated through said chamber.
 18. Apparatus for treating a substrate without exposure to a high current density comprising: electrically insulative means defining a gastight chamber, the pressure within said chamber being less than the pressure externally of said chamber; means including first and second electrodes disposed exteriorly of said chamber for generating a high frequency induced plasma in said chamber without generating a plasma externally of said chamber whereby the current flowing between said electrodes is reduced; and, means for passing a substrate to be treated through said chamber. 